I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to mobile station location within a wireless communication system.
II. Description of the Related Art
Wireless communication systems are becoming commonplace in modern society. One of the most scarce and complex elements of a wireless communication system is the wireless spectral bandwidth over which the wireless link operates. In order to efficiently use the segmented spectral bandwidth, modern commercial cellular communication systems are designed to reuse the allocated spectral resources. Spectral reuse is achieved by allocating the same spectral resources to multiple coverage areas spaced apart by some minimum distance. Depending on the type of system, the spectral resources may be allocated into discrete channels in the time, frequency or code domain or a combination of these. In each case, at some distance away from a first coverage area using a channel, a second coverage area reuses the same channel. The distance between the first and second coverage areas is chosen such that the signal levels emanating from the first coverage area have become sufficiently small at the second coverage area that co-channel interference between the first and second coverage areas is below an acceptable threshold level.
All cellular systems employ a power control mechanism to some extent. By using a power control mechanism, the mobile station changes its transmit signal level to compensate for variations in the path loss between the base station and the mobile station. In an ideal cellular system, the mobile station controls its power level so that within a great majority of the service area, its signal arrives at only one base station with a signal to noise ratio large enough for efficient demodulation.
In wireless communication systems, the actual location of the mobile station is often unknown. Some information about the general location of the mobile station can be easily determined. For example, in a cellular system, each base station defines a physical coverage area. In an ideal cellular system, the coverage areas of adjacent base stations abut one another to create a continuous service area. Mobile stations located within the coverage area of a base station may establish communication with the base station. When a mobile station moves past the outer boundary of the coverage area of a first base station into the coverage area of a second base station, communication with the first base station may become greatly compromised or even impossible due to the decreased signal to noise ratio at which the mobile station signal is received at the first base station. In actual systems, the coverage area of adjacent base stations overlap to some extent. Within the coverage area overlap region, a mobile station may establish communication with either base station or both base stations, depending on the communication techniques used. The coverage area overlap region can be used to execute handoff from one base station to another so that a continuous connection is provided to the mobile station as it moves about within the service area. In this way, the position of a mobile station can be limited to the coverage area corresponding to the base station which is currently providing service to the mobile station. However, base station coverage areas can be relatively large such as several miles in diameter. In many emergency situations, such imprecise location information is nearly useless.
Several techniques have been developed in order to aid in locating a mobile station in a wireless system using one or more location parameters. For example, the angle of arrival, time of arrival and time difference of arrival have all been used as location parameters to determine the location of a mobile station in a wireless system.
FIG. 1 is a diagram illustrating an angle of arrival for a wireless mobile station 102. The angle of arrival is an indication of the direction from which the signal from the mobile station 102 is received at the base station 100. The measured angle of arrival designates the location of the mobile station on a line of bearing. The angle of arrival does not provide any information about the distance between the base station 100 and the mobile station 102.
An interferometer is one means by which the angle of arrival may be measured. An interferometer determines the angle of arrival based on a phase difference of the signal arriving at two or more antennas elements.
A beam forming method can also be used to measure the angle of arrival. A beam forming method determines the angle of arrival based on a best match between the amplitude and phase response of an antenna array and the amplitude and phase signal measurements for each antenna element of the array.
Super-resolution techniques may also be used to determine the angle of arrival. Super-resolution techniques determine the angle of arrival based on a determination of the multipath model and signal statistics, not knowing the signal itself. Academic efforts refining these and other techniques are currently in progress.
The time of arrival determines a circle upon which the mobile station 102 may be found with relation to the base station 100 as shown in FIG. 2. The radius of the circle is calculated by multiplying the delay between the transmission of a signal from the mobile station 102 and reception of the signal at the base station times the speed of light. (radius=c*delay). The time of arrival does not provide any information concerning the angle of the mobile station 102 with respect to the base station 100. In a typical prior art system, the base station 100 sends a signal to the mobile station 102 which repeats the signal as soon as it is received. The delay between the transmission and reception of the signal is determined by cross-correlating the transmitted signal and the received signal at a series of time offsets. The cross-correlation exhibits a peak at the time offset equal to the round trip delay or twice of the one-way transmission delay time. The transmitted signal is often called a reference signal. A reference signal is only available when the base station has a priori knowledge concerning the data content of mobile station signal.
FIG. 3 is a diagram illustrating a time difference of arrival determination for a wireless mobile station 102 with respect to a base station 100A and a base station 100B. The difference in the time of arrival of a signal received at two base stations determines a hyperbola upon which the mobile station 102 is located. In a typical prior art system, the mobile station 102 transmits a signal which is received by both base stations 100A and 100B. Each base station determines the absolute time at which the signal is received. By comparing the difference, a relative distance between the two base stations is determined. For example, by comparing the time difference of arrival, the location of the mobile station 102 is determined to be about 1 kilometer closer to the base station 102B than the base station 102A. Alternatively, received signals from base stations 100A and 100B are brought to a common processing site where they are cross-correlated to yield directly the difference in propagation time from the mobile station to the base stations.
As a signal propagates between the base station and the mobile station over the wireless channel, it is attenuated by and reflects from objects in the field. At the base station, the various reflected propagations are offset in time from one another due to the differences in the path lengths which the signals travel. For example, FIG. 4 is a diagram showing three different propagation paths of a signal transmitted by the mobile station and received by the base station. Typically, the first signal to arrive has the largest amplitude and travels a relatively direct path from the mobile station to the base station. However, this is not always the case, especially when no viable direct path is available. In addition, depending on the phase at which the multipath signals arrive at the base station, the multipath signals may add destructively. The destructive addition of multipath signals may result in signal fading meaning that the combined signal strength of two or more propagation paths is less than the sum of the signal strength of each signal alone.
Multipath is the major source of error in the measurement of an angle of arrival, time of arrival or time difference of arrival location parameters. Multipath distorts the result of the cross-correlation as well as the angle detection processes, thus, limiting the accuracy of the location parameter determination. The limited accuracy of the location parameters inhibits an accurate determination of the exact position of the mobile station. For example, when no viable direct path from the mobile station to the base station is available, the resulting round trip delay measurement reflects a greater distance than the actual distance between the mobile station and the base station. Also, when no viable direct path from the mobile station to the base station is available, the angle of arrival measured by the base station reflects one of the multipath angles of arrival which is seldom reflective of the actual location of the mobile station. Even when a direct path is available, the presence of the multipath signals may distort the detection process causing errors in the measured location parameters.
In addition to multipath, the accuracy of the location parameters measurements is limited by the signal to noise ratio at which the signals are received at the base station. Under ideal conditions of additive white Gaussian noise (AWGN), the variance of the accuracy of the location parameters is limited by the Cramer-Rao bound. In deployed systems, the noise is actual a combination of thermal AWGN, co-channel interference from other mobile stations using the same channel and spurious interference from other man-made sources.
Using two or more of measurements of the location parameters discussed above, a more precise location of the mobile station may be determined. Assuming variations in height within the area of interest do not constitute a serious problem requiring an increase in the number of location parameters measurements, the location of the mobile station may be determined according to Table I.
TABLE I Location Parameters Minimum No. of Base Measured Stations Angle of Arrival and 1 Time of Arrival Angle of Arrival and 2 Time Difference of Arrival Angle of Arrival 2 Time of Arrival 2 Time Difference of 3 Arrival
The mobile station location can be determined based on measuring the angle of arrival and the time of arrival at one base station. For example, imagine superimposing FIG. 1 over FIG. 2. The intersection of the line of bearing with the circle determines the location of the mobile station.
The mobile station location can be determined by measuring the angle of arrival at a first base station and the time difference of arrival between a first and a second base station. For example, imagine superimposing FIG. 1 over FIG. 3. The intersection of the line of bearing with the hyperbola determines the location of the mobile station.
The mobile station location can be determined by measuring the angle of arrival from two base stations as shown in FIG. 5. In FIG. 5, both the base station 100A and the base station 100B determine an angle of arrival for the mobile station 102. The intersection of the lines of bearing defined by the measured angle of arrivals determines the location of the mobile station 102.
The mobile station location can be determined by measuring the time of arrival from two base stations as shown in FIG. 6. In FIG. 6, both the base station 100A and the base station 100B determine a time of arrival for the mobile station 102. The two intersection points of the circles defined by the measured time of arrival radii determine two possible locations of the mobile station 102. Obviously some other mechanism must be used to eliminate one of the possible location points in order to precisely determine the mobile station location.
In a similar manner, the mobile station location can be determined based on the measuring the time difference of arrival at three base stations as shown in FIG. 7. In FIG. 7, the base stations 100A, 100B and 100C determine a time difference of arrival for the mobile station 102. The two intersection points of the resulting hyperbolas determine two possible locations of the mobile station 102. However, in most cases, one of the intersection points is far removed from the actual coverage areas of the three base stations and may be easily identified as an erroneous location.
The error in location determination, based on the minimum number of measurements which are required according to the various techniques shown in Table I, may be quite large even when the error in the measurements is relatively small. For this reason, benefits can obtained by combining techniques and making additional measurements. The additional information gained through the additional measurements reduces the error in location determination caused by errors in the location parameter measurements. The accuracy of a location system may be significantly improved by providing measurements from additional base stations, even when the additional measurements are also subject to error. For example, in FIG. 8, angle of arrival and time of arrival measurements are performed at two base stations 100A and 100B. From this information, four different estimates of the location of the mobile station 102 may be determined. By combining the results, a more accurate determination of the position of the mobile station 102 may be determined. More information concerning one such system may be found in U.S. Pat. No. 5,592,180 entitled "DIRECTION FINDING AND MOBILE LOCATION SYSTEM FOR TRUCKED MOBILE RADIO SYSTEMS."
However, in most cases, practical conditions impose a threshold on a minimum signal to noise ratio at which the mobile station signal is received at the base station in order for the signal to be detected and a meaningful location parameter to be measured. Base stations which do not receive the mobile station signal with at least the threshold signal to noise ratio cannot provide useful location parameter measurements. The signal to noise ratio at which the mobile station signal reaches the base station is a function of the distance between the base station and the mobile station as well as other factors. As a mobile station moves about within a cellular system, typically only one base station at a time receives the mobile station signal at a relatively high signal to noise ratio. The remaining base stations receive the mobile station signal at a relatively low signal to noise ratio. The low signal to noise ratio affects the accuracy of the location parameters measured by these base stations or prevents measurement altogether. In turn, the poor location parameters yield faulty location determinations or no location determinations when the minimum number of required location parameters are unavailable.
Therefore, there is a need in the industry to provide an extended measurement range and more accurate position location within a cellular system.